The present invention relates to methods for forming pillars and proppant-free channels in propped fractures.
Subterranean wells (e.g., hydrocarbon producing wells, water producing wells, and injection wells) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulate solids, such as graded sand, bauxite, ceramics, or even nut hulls, are suspended in a treatment fluid and then deposited in the fractures. These particulate solids, or proppant particulates, are generally deposited in the fracture in a concentration such that they form a tight pack of particulates, or “proppant aggregates,” which serve to prevent the fractures from fully closing once the hydraulic pressure is removed. As used herein, the term “proppant aggregate(s)” refers to a coherent body, such that when the aggregate is placed into the fracture(s) or into a fracturing or treatment fluid, the aggregate should not become dispersed into smaller bodies without the application of shear. By keeping the fracture from fully closing, the proppant particulates aid in forming conductive paths through which fluids may flow.
An alternative fracturing approach involves placing a reduced volume of proppant particulates into a fracture by forming consolidated, proppant aggregates which can create a propped fracture having high porosity, permeability, and/or conductivity. Proppant aggregates tightly pack and abut the face of the fracture to create proppant pillars when the fracture closes after the hydraulic pressure is removed. Typically, proppant aggregates do not become dispersed into smaller bodies without the application of shear. However, during fracturing operations, proppant aggregates may experience shear force or stress that can cause the proppant aggregates to disperse into smaller bodies. If these small bodies do not re-form into aggregates, they may not be of sufficient size to prop open a fracture. In some cases, partial fracture closure may result if the proppant aggregates do not remain a coherent body.
One way to compensate for the potential of proppant aggregate dispersion due to downhole shear force or stress is to pump the proppant aggregates into the fracture in viscous gel treatment fluids. While gel treatment fluids afford some protection to the proppant aggregates from dispersion, treatment fluids are typically shear-thinning fluids. As a result, when the treatment fluid encounters downhole shear force or stress (e.g. high impact areas during injection, elbows, perforations, and the like), the fluid's viscosity decreases, thus the protection afforded to the proppant aggregates also decreases. Therefore, the shear-thinning property of most treatment fluids only provides protection to proppant aggregates when the fracturing operation will cause the treatment fluid to experience only trivial amounts of shear force or stress. However, in typical fracturing operations, the treatment fluid will experience greater than trivial shear force or stress, which may result in proppant aggregate dispersion and compromised fracture conductivity.
The degree of success of a fracturing operation depends, at least in part, upon fracture conductivity once the fracturing operation has ceased and production commenced. Therefore, a practical method of reducing proppant aggregate dispersion during fracturing operations may be of value to one of ordinary skill in the art.